The present invention relates generally to the casting of metal alloys and more specifically, to the continuous casting of ferroalloys and slags into shapes for subsequent use as remelt alloys or as deoxidation additions in iron and steel making operations or like metallurgical processes.
In recent years, commercial users of ferroalloys and deoxidizing agents have increasingly specified that these products be supplied in relatively uniform cross-section for ease in handling by mechanized equipment and to insure uniform melting rates. For example, a material such as ferrosilicon is often specified by commercial users to be supplied in lumps with dimensions no greater than 75 mm, nor less than 25 mm.
Heretofore, in the conventional manufacture of ferro-alloys, it has been common practice to cast a large plate or block of the material in iron moulds with the cast product having gross dimensions of approximately 800-1200 mm.times.800-1200 mm.times.25-150 mm. These moulds are usually mounted on a turntable device, holding ten or more such moulds, which revolves in a horizontal plane to permit filling of each moulds in sequence from a ladle, or the moulds are placed on the floor and cast either individually or by allowing metal to overflow in cascade fashion to two or more moulds arranged in steps.
Due to the fact that slags obtained in primary melting operations oftentimes contain valuable metallic elements which can be recovered by subsequent processing, it is also desirable to cast the slag into shapes which lend themselves to further reclamation operations. As such, slag is oftentimes cast in a similar manner as the ferroalloys.
After the cast ferroalloy or metallic slag material has cooled for a period of time, which may range from 30 minutes to 8 hours, the cast blocks, weighing upwards of one ton, are removed from the moulds and taken to a motorized crushing device where they are broken-up into smaller pieces. The crushed product is separated by size in a screening operation to eliminate the oversized and undersized particulate material. The properly sized material is then packed for shipment with the oversized material returned for additional crushing and screening while the undersized material, which is not lost or too fine, may be returned for remelt. This common production process, thus, entails high energy consumption in the crushing operation, coupled with a relatively high yield loss due to off-sized material and through the generation of fines. In addition, in the case of silicon alloys, these powder-like fines represent a toxic waste problem involving the dust collection and the resultant disposal thereof.
In addition to the high expense of operating and maintaining a crusher, it is not uncommon to generate product losses during the crushing and screening operations of over 30% of the quantity originally cast. Another major problem experienced is that since the temperature of the cast iron moulds is kept above 500.degree. C. for extended periods, the moulds are subject to thermal stress cracking, resulting in expensive and frequent replacement of the moulds. A further problem which results from the slow cooling of large alloy castings is the metallurgical segregation of the various metallic elements which may cause a lack of chemical uniformity in the cast product. Portions of the casting which solidify first may contain a different alloy concentration than the last to solidify, in the central regions of the casting. Upon crushing, there may be a slight non-uniformity in the chemical composition occurring among the individual crushed particles.
It can, therefore, be appreciated that a significant advantage can be obtained by casting material directly to a desired shape and eliminating the crushing and screening operations. Prior attempts to produce ferroalloys and slag in this manner have been unsuccessful since the service life of the iron moulds has been relatively short so as to render the process uneconomical.
An attempt to cast ferroalloy shapes is shown in U.S. Pat. No. 3,429,362 which discloses a method of producing cast ferroalloy shapes with indented grooves or connecting "necks" between adjacent cavities to control breakage to a predetermined size after cooling in still air for about 6 minutes. Such a slow cooling practice ignores the threat of heat checking in iron moulds and resultant problems of mould deterioration. U.S. Pat. No. 3,581,809 discloses a device for casting ferroalloys comprising a rotary cylinder enclosed in a protective atmosphere and having a plurality of cavities on its outer surface. The problem of mould deterioration is apparently avoided through the use of expensive, graphite or water chilled copper moulds which can withstand prolonged periods at elevated temperatures without thermal degredation.